The Princess of the Crystal is the entity that uses Himari's body as a host. She demands the Penguin Drum as payment for extending Himari's life. Later on it is revealed the entity is one of Momoka's soul halves, now possessing Himari in order to extend her life, and indirectly guide Kanba and Shoma to their destiny paths to save Himari.
She is blunt and doesn't like to deal with defiance of any sort unlike Himari and calls Kanba and Shoma lowlifes. When she possesses Himari, her eyes turn into a bright shade of pink, and we see her wearing an elaborate dress.
Despite using Himari's body, she herself says "I am not your sister" to Kanba and Shoma and never answers to Himari's name. She demonstrates to the brothers that she can drink cow's milk in Himari's body, even though Himari herself is allergic to it.
Since the Princess of the Crystal is one of Momoka's halves this would be considered her darker half. A serious side shown only once when she faced Sanetoshi and told him she'd "Banish him from the world and blow him into oblivion." This language is similar to the Princess of the Crystal's speech.
Mario is possessed by the lighter half (the side shown with Tabuki & Yuri), and that side is known as the "Prince of the Crystal" judging by his outfit, although not much of that side was seen.